1. Field of the Invention
The present invention relates to an automotive antenna system for diversity reception.
2. Description of the Prior Art
It is difficult to receive radio waves steadily in a moving automobile at all times. Waves utilized in FM broadcast belonging to high frequency bands known as VHF bands have strong straight-going properties. Direct waves tend to interfere with waves reflected by buildings or mountains to create multi-path noise.
An automotive antenna system for diversity reception has ben proposed to improve the directional pattern and to reduce the multi-path noise such that radio waves can well be received at all times irrespective of the direction of the vehicle body relative to the incoming waves. Such an automotive antenna system comprises two pole antennas having their directivities, which are mounted on the vehicle body. The output from each of the pole antennas is selectively supplied to the input of a high-frequency amplifier. This high-frequency amplifier has its rearward stage connected with an intermediate amplifier which in turn is adapted to generate voltages proportional to the electric field intensity. These voltages are compared and calculated such that radio waves can steadily be received by one of the two pole antennas which has an increased intensity of electric field with less disturbance from multi path noises.
In the prior art diversity reception type automotive antenna systems, each utilizing two directional pole antennas, therefore, the outputs of the pole antennas were varied through an increased extent because of variations in the electric field and of the directivities of the pole antennas. Thus, the pole antennas were more frequently changed from one to another to generate noise on variations of S/N ratio.